Bugfix : Set Concentrator ClientThumbprint as nullable

[audserraCGI]

Description

What's new?

• Closes Bug: Database exceptions on devices sync job when processing concentrators #1533

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Tests
• Other

[codecov]

Codecov Report

Base: 89.70% // Head: 89.70% // No change to project coverage 👍

Coverage data is based on head (ab9007e) compared to base (3d9e857).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1537   +/-   ##
=======================================
  Coverage   89.70%   89.70%           
=======================================
  Files         256      256           
  Lines        8011     8011           
=======================================
  Hits         7186     7186           
  Misses        825      825           

Impacted Files	Coverage Δ
...AzureIoTHub.Portal.Domain/Entities/Concentrator.cs	100.00% <100.00%> (ø)
...rtal.Shared/Models/v1.0/LoRaWAN/ConcentratorDto.cs	100.00% <100.00%> (ø)

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